This is a competing continuation application for DK55023. Both Types 1 and 2 diabetes result from inadequate production of insulin. A major goal in diabetes research is the development of a means to induce pancreatic beta cell replication. We have demonstrated that adult rodent and human beta cells can be induced to replicate by overexpression of the growth factors, placental lactogen (PL), hepatocyte growth factor (HGF), and parathyroid hormone-related protein (PTHrP), and also by adenoviral transduction with cyclin D1 and cdk-4. More recently, we have completed a comprehensive analysis of 20 cell cycle regulatory molecules at the key beta cell replication checkpoint, G1/S, in the murine islet. We have found that induction of proliferation in beta cells by HGF and PL is associated with a dramatic and unique upregulation of the cell cycle inhibitor, p21. p21 is also increased by PTHrP and by induction of mitogenesis by cyclin D1 and cdk-4. Moreover, p21 is also increased in human beta cells by activation of the cell cycle, and shifts from a cytoplasmic to a nuclear location on induction of cell cycle progression. Further, islets from p21-null mice display a more robust proliferative response than normal islets to HGF and prolactin, a PL surrogate, indicating that p21 serves as a molecular "brake" on cell cycle progression in the beta cell. Finally, another member of the p21 family, p27, has recently been shown by others to inhibit beta cell cycle progression in obese rodent models, indicating that the p21/p27 family may be central to cell cycle control in the pancreatic islet. Accordingly, we plan the following four Specific Aims: 1. To determine the consequences of p21 underexpression in the islet. 2. To explore the residual cell-cycle restraints on proliferation in p21 and p27 KO mice. 3. To determine the consequences of p21 overexpression in the beta cell. 4. To define the regulation, trafficking and partners of p21 and p27 in the islet. These studies should clearly define the mechanisms that control p21 and p27 expression in the beta cell, and will clarify the therapeutic potential for maneuvers that regulate p21 and p27 in the treatment of diabetes.